Meet the shell

Welcome aboard Kestrel Station, a research outpost in low orbit. Every system up here - life support, cargo robots, the coffee machine - runs on Linux, and the crew talks to all of it through one tool: the shell. You’re the newest engineer, and by the end of this track you’ll be the one writing the scripts that keep the station humming.

Three pieces work together whenever you “use the command line”:

• The terminal (Terminal, iTerm2, Windows Terminal, the panel in VS Code) is just a window that shows text and sends your keystrokes.
• The shell is the program running inside it. It reads what you type, works out what you mean, runs programs, and shows their output.
• Commands are the programs (and shell built-ins) that do the actual work: ls lists files, grep searches text, git manages code.

Bash - the Bourne Again SHell - is the most widely used shell. It’s the default on most Linux servers, in Docker containers and CI pipelines, and it’s one install away on macOS (whose default is now the very similar zsh) and Windows (Git Bash or WSL). Everything you learn here carries over to almost every shell you’ll meet.

When the shell is ready, it shows a prompt - often something like ada@kestrel:~$. The $ means “type here”. (A # usually means you are the all-powerful root user, so be careful.) In lessons, a leading $ marks what you type; don’t type the $ itself.

A command line is a list of words separated by spaces. The first word is the command; the rest are passed to it:

1ls -l -a /etc
2│  │  │  └── argument: what to work on
3│  └──┴───── options (flags): how to do it
4└─────────── command

Short options start with one dash and can usually be combined (ls -la is ls -l -a). Long options start with two dashes and are spelled out (ls --all). Spaces matter: ls -la lists files, while ls - la asks for two files named - and la.

echo prints its arguments separated by single spaces, then a newline. printf takes a format string first and fills its %s (string) and %d (whole number) placeholders with the remaining arguments - and it only adds a newline where you write \n. These examples run for real: press Run, then Edit and experiment.

Notice line 3: the shell splits the line into words at any run of spaces before echo ever sees it, so echo receives five tidy words and joins them with single spaces. Quotes keep text together as one word. You’ll lean on that idea constantly. In printf, %-10s pads a string to 10 characters, left-aligned, and %5s right-aligns it in 5 - perfect for tables.

Nobody memorizes every option. Experienced engineers look things up constantly - from the terminal itself:

Some handy keys: Tab completes names, ↑ recalls earlier commands, Ctrl+R searches your history, Ctrl+C cancels the running command, and Ctrl+L clears the screen.